Such devices are known, in particular from DE 196 42 472, DE 197 05 767, DE 197 05 768 and DE 199 19 602. In the devices of this type, balancing the magnetic fields generated by the two windings, primary and secondary formed on a common core of ferromagnetic material, involves interlocking the current powering the secondary winding with a zero value of the resultant magnetic field detected by the means sensitive to this field. When the resultant field is zero, the current to be measured and the current in the secondary winding are in inverse ratio to the number of turns of these windings. A measurement of the intensity of the current flowing in the secondary winding, carried out when the fields are in balance, can therefore be used to achieve the intensity of the current to be measured. Thus, perfect electrical insulation is ensured between the circuit in which the current to be measured is flowing and the circuit in which the balancing current is flowing.
In the devices of this type, the means sensitive to the resultant field is most commonly formed by a linear Hall effect probe, positioned in an airgap of the ferromagnetic core, for example. Such a probe delivers an electrical signal representative of both the direction and the intensity of the flux of the resultant field in the airgap. To detect cancellation of this flux, detection upon which the operation of the device depends, the signal is processed in a comparator and, with the help of a clock, a pulse width modulated signal can be formed, specifically to control the power supply of the secondary winding.
Drifts on such a device, due to thermal and mechanical stresses such as those routinely affecting electronic devices on board motor vehicles for example, must therefore be compensated by additional electronic means which add to the production cost of the device. A solution to this problem could be to use a linear, programmable Hall effect probe, which commonly incorporates means of providing the necessary compensations for thermal and/or mechanical drifts. This solution is, however, also hampered by the high cost of these programmable probes. It is therefore unsuitable for mass productions intended for a wide customer base, which need to be produced at the lowest possible costs, as in the case in particular of the electronic circuitry on board motor vehicles.